Fluidized bed reactors, such as circulating fluidized bed reactors, are used in a variety of different combustion, heat transfer, chemical and metallurgical processes. Typically heat, originating from combustion or other exothermic processes, is recovered from the solid particles of the fluidized bed by using heat transfer surfaces. Heat transfer surfaces conduct the recovered heat to a medium, such as water or steam, which transfers the heat from the reactor.
The heat transfer surfaces are typically located in the processing chamber or within a convection section arranged in the gas pass after the processing chamber or, in circulating fluidized bed reactors, within a particle separator. Additional heat transfer surfaces are often arranged in a separate heat transfer chamber (HTC), which may be a part of the processing chamber, a separate chamber adjacent to the processing chamber or, in circulating fluidized bed reactors, part of the solid particles recycling system.
An HTC is typically a bubbling fluidized bed, which comprises inlet means for introducing a continuous flow of hot solid particles from the processing chamber into the HTC, heat transfer surfaces, and outlet means for continuously recycling solid particles discharged from the HTC into the processing chamber.
Corrosion is a factor which must always be taken into account when designing heat transfer surfaces. It is especially important when the heat transfer surfaces are in a fluidized bed reactor utilized in processes which use or produce corrosive materials. An example of such is burning difficult fuels, such as straw or RDF, which contain highly corrosive impurities, e.g., chlorides. Corrosive impurities are then also present in the fluidized bed material, and thus come into contact with the heat transfer surfaces in an HTC, leading to rapid corrosion of said surfaces. For example, chlorine in the bed material may cause chloride corrosion on the heat transfer surfaces.
Corrosion problems are especially severe when the temperature in an HTC is high, e.g., due to afterburning, which may easily take place when the HTC is directly connected to the furnace. Afterburning or other chemical processes in an HTC can also lead to a reducing atmosphere, where Co-corrosion easily takes place. Reducing conditions together with chloride deposits are especially susceptible to increased corrosion attack.
Corrosion and erosion based wastage of metals is an essential problem in all bubbling fluidized beds, and many efforts have been made to minimize it. Normal remedies against corrosion are changes in the metal surfaces and their temperatures. Surface treatments, such as chromising, nitriding, or coating with tungsten carbide are in some cases effective. Because all corrosion mechanisms are temperature dependent, corrosion of the heat transfer surfaces can to some extent be avoided by locating the surfaces at appropriate positions in the system.
However, surface treatments are not always feasible, as conditions and temperatures may vary at different locations and stages of the processes. Also, when choosing operating temperatures, the corrosive impurities present in each specific system have to be taken into account. These impurities may vary when using different parameters, such as different fuels, in the process. Therefore, procedures to minimize the risk of corrosion by reducing the concentrations of the actual corrosive impurities are highly wanted.